The present invention relates to a technique of ejecting ink to create dots and print a multi-tone image. More specifically the present invention pertains to a printing technique using a plurality of different dots created in different states with a substantially e equivalent quantity of ink.
A diversity of printers have widely been used as output devices to print multi-color, multi-tone images processed by the computer. One of such printers is an ink jet printer that creates dots with several color inks ejected from a plurality of nozzles provided on a print head, so as to record an image. The ink jet printer generally enables expression of only two tones, that is, a dot-on state and a dot-off state, in each pixel. The ink jet printer accordingly carries out the halftone processing, which expresses multiple tones of original image data by a distribution of dots, prior to printing an image.
Multi-valued printers that enable expression of at least trinary tones in each pixel are one of the proposed techniques to attain richer tone expression. The multi-valued printers include printers using a plurality of inks having different densities with regard to an identical hue and printers using variable quantities of ink to create dots. The variable-ink quantity printers include printers that change the frequency of ink ejection to vary the quantity of ink in each pixel and printers that vary the quantity of ink per ejection. Such multivalued printers ensure the smooth tone expression and improve the image quality.
In the ink ejection-type printers, the image quality of the resulting printed image is affected by the printing paper. This is because the state of penetration of ejected ink depends upon the printing paper. For example, in the case of plain paper, ink readily penetrates into the sheet. The plain paper is not able to sufficiently hold the ink dye in the vicinity of the sheet surface and may thus not ensure the desired density expression. In order to compensate for this potential disadvantage, the prior art technique increases the quantity of ink ejection than usual in the case of printing in a printing medium of high ink permeability, for example, the plain paper. A concrete procedure changes the contents of the halftone processing to enhance the dot recording density when such a printing medium is selected.
The prior art multivalued printer has a relatively restricted density range expressible in each pixel. The structure using a large number of different inks having different densities to express a greater number of tones disadvantageously expands the size of the print head. The printing medium generally has an upper limit in quantity of ink absorbable per unit area (hereinafter referred to as the duty restriction). The variation in quantity of ink ejected in each pixel is accordingly restricted to the upper limit. The printing medium having high ink permeability, for example, the plain paper, has a relatively low duty restriction. The prior art printing apparatus does not attain the sufficient density expression nor ensure the sufficient image quality in such printing media.
The arrangement of creating dots with the varying quantity of ink also has the restrictions by the printing speed and the print head mechanism. Under the condition of a fixed driving frequency of the print head, the printing speed is lowered with an increase in frequency of ejection per pixel. The allowable range of varying the quantity of ink ejected from each nozzle has upper and lower limits according to the nozzle diameter. With a recent trend of high printing resolution that requires very fine dots, the allowable range of varying the quantity of ink is more strictly restricted.
In the prior art technique, the area of high density is expressed by raising the dot recording density or by increasing the quantity of ink ejected in each pixel. This increases the quantity of ink ejected per unit area and may cause stains or blots.
Because of these factors discussed above, the prior art technique has a relatively restricted range of tones expressible in each pixel.
One object of the present invention is to extend a range of tone values expressible in each pixel and thereby improve the image quality in a printing apparatus that ejects ink to print an image. Another object of the present invention is to provide a print head that attains such a wide tone range and a method of driving the print head. Still another object of the present invention is to ensure adequate tone expression in the case of printing in a printing medium having a high penetration rate of ink.
At least part of the above and the other related objects is attained by a print head that pressurizes ink in an ink conduit, through which a supply of ink is fed from an ink tank to a nozzle, so as to cause ink to be ejected from the nozzle and create a dot.
The print head includes: a pressure variation unit that varies a pressure applied to the ink in the ink conduit; and a driving unit that controls the pressure variation unit to apply the pressure to the ink along a preset pressure waveform.
In this print head, the driving unit varies a parameter relating to a pressure reduction, so as to enable different dots to be created in different dot creation states with a substantially identical quantity of ink.
In the print head of the present invention, the driving unit varies the parameter relating to the pressure reduction, so as to enable different dots to be created in different dot creation states with a substantially identical quantity of ink.
In the print head of this configuration, varying the waveform of the pressure applied o the ink enables different dots to be created in a variety of dot creation states with a substantially identical quantity of ink. Under the condition of ejecting a fixed quantity of ink, the different dot creation states vary the density expressed. The print head of the present invention accordingly varies the density expressed in one pixel under the condition of a substantially identical quantity of ink. Printing with the print head of the present invention ensures the richer tone expression and thereby improves the image quality. This arrangement extends the expressible tone range without increasing the quantity of ink, thus reducing the occurrence of blots or stains.
The relationship between the dot creation state and the expressed density is described below. The dot creation state represents the shape of dots actually created on the printing medium when a substantially identical quantity of ink is ejected. Ejection of ink at one point in a concentrated manner and ejection of ink in a predetermined area in a diffused manner result in creating dots of different forms. The substantially identical quantity of ink does not require strict constancy in the plural dot creation states, but may be in a range that can be regarded as constant based on the quantity of ink absorbable by the printing medium. It is conventionally thought that different dot creation states express an identical density as a whole in the case of a fixed quantity of ink ejection.
As results of minute analyses, the inventors of the present invention have, however, found that different dot creation states vary the total area of dots. The variation in total area of dots naturally varies the density expressed as a whole.
The principle of varying the area according to the dot creation state is described with the comparison between the case of creating a single dot and the case of creating a split dot. FIG. 1 shows a dot created by ejecting ink at one point in a concentrate manner. The upper row of the drawing shows a moment when an ink droplet Ip hits against a printing medium P. The ink droplet Ip penetrates in the direction of the depth of the printing medium P at a velocity Vy and in the direction of the plane at a velocity Vx. The penetration results in creating a single dot Dt having a diameter d as shown in the lower row of the drawing. The ejected ink droplet penetrates into the printing medium to have a sectional shape defined by the hatched area in the drawing.
FIG. 2 shows dots created by ejecting ink in a splitting manner as two ink droplets Ip1 and Ip2. The upper row of the drawing shows a moment when the ink droplets Ip1 and Ip2 hit against the printing medium P. In this example, the ejected ink is split into the ink droplets Ip1 and Ip2 of an identical size. The total quantity of the ink droplets Ip1 and Ip2 is identical with the quantity of the ink droplet Ip in the example of FIG. 1.
When ink is ejected in a splitting manner, the respective ink droplets Ip1 and Ip2 penetrate in the direction of the depth of the printing medium P at the velocity Vy and in the direction of the plane at the velocity Vx as in the case of FIG. 1. The penetration results in creating dots Dt1 and Dt2 having a diameter d1 as shown in the lower row of FIG. 2. Each ejected ink droplet penetrates into the printing medium to have a sectional shape defined by the hatched area in the drawing. The diameter d1 is smaller than the diameter d.
The penetration speed of the ink droplet Ip1 (FIG. 2) into the printing medium P is equivalent to that of the ink droplet Ip (FIG. 1). Namely the ink droplets penetrating in the printing medium have similar shapes (the hatched areas in FIGS. 1 and 2). As mentioned above, the volume of the ink droplet Ip1 is half the volume of the ink droplet Ip. The similarity ratio of the dot Dt1 to dot Dt, that is, the ratio of the diameter d1 to the diameter d, is given as a cube root of the volume ratio. In this example, the volume of the dot Dt1 is 0.5 times as large as the volume of the dot Dt, so that the relationship between the diameter d1 and the diameter d is defined by Equation (1) given below:
D1=0.5(⅓)xc3x97dxe2x80x83xe2x80x83(1)
The areas of the resulting dots Dt and Dt1 are respectively proportional to the square of the diameters d and dt1. The relationship between area of the dot Dt1 and the area of the dot Dt is accordingly defined by Equation (2) given below:
Dt1=0.5(⅔)xc3x97Dtxe2x80x83xe2x80x83(2)
In the example of FIG. 2, two dots of the same area are created. The total area of the dots created in the example of FIG. 2 is accordingly given by:
Dt1+Dt2=2xc3x970.5(⅔)xc3x97Dt≈1.26Dt
Namely the method of splitting the dot into two divisions enhances the resulting expressed density to approximately 1.26 times. The above description regards the case of splitting the dot into two divisions. Splitting into a greater number of divisions further enhances the resulting expressed density. When A1 and An respectively denote the area of a single dot and the area of each dot division by splitting a dot into n divisions, their relationship is defined by Equation (3) given below, based on the same principles as those of Equations (1) and (2) discussed above:
An=(1/n)(⅔)xc3x97A1xe2x80x83xe2x80x83(3)
FIG. 3 is a graph showing the relationship between the number of dot divisions and the total dot area. The total dot area is calculated according to Equation (3) by changing the number of splits from 1 to 4. As clearly understood from the graph, the total dot area by a fixed quantity of ink increases with an increase in number of splits. Since it is thought that the resulting expressed density is substantially proportional to the dot area, the resulting expressed density is heightened with an increase in number of splits.
For example, when a single dot is created with a quantity of ink q1, the total dot area is equal to Ar1 in the graph. When a split dot having two divisions is created with the quantity of ink q1, on the other hand, the total dot area is equal to Ar2 in the graph. This is equivalent to the density when a single dot is created with a greater quantity of ink q2. The quantity of ink q2 is approximately 1.4 times as large as the quantity of ink q1. The method of splitting a dot into a plurality of divisions readily enhances the resulting expressed density to the level attained by significantly increasing the quantity of ink.
The above description is on the assumption that the dot is completely split into two divisions. The two dots may alternatively be created in a partly overlapping manner. Namely the dot may not be split but may be deformed in shape. In such cases, the size of the overlapped portion determines the total dot area and the resulting expressed density.
As described above, the print head of the present invention enables dots to be created in a variety of dot creation states. It is preferable that the different dot creation states have different numbers of dot divisions. As shown in the graph of FIG. 3, varying the number of dot divisions ensures a significant difference in resulting effects. The number of dot divisions may be set arbitrarily. It is, however, desirable to create a nonsplit dot and a split dot having two divisions since these dots are created most stably.
Since it is conventionally thought that a fixed quantity of ink ejection gives the same resulting expressed density, no techniques have been proposed to change the dot creation state. It is known in the art that splash of ink at the time of ink ejection sometimes forms very small dots called satellites in the vicinity of a target dot to be created. No studies have, however, been performed to examine the effects of the satellites on the resulting density. The occurrence of satellites is generally regarded as undesirable, and no prior art techniques have tried to change the dot creation state by positively forming satellites.
The technique of the present invention changes the parameter relating to the pressure reduction, so as to control the dot creation state. The print head of the present invention ejects ink by varying the pressure applied to the ink in the ink conduit. The ink is ejected under a high pressure of or above a preset level. Based on the results of detailed experiments, the inventors of the present invention have found that the arrangement of setting a pressure reducing period at least one of before and after a pressure raising period and varying the pressure reducing conditions effectively changes the dot creation state without requiring a variation in quantity of ink.
The relationship between the pressure waveform and the dot creation state is described here in the case where the preset pressure waveform includes a high pressure section to apply a high pressure to the ink and a subsequent reducing pressure section to subsequently reduce the pressure.
In this case, any of the following factors may be applied for the parameter relating to the pressure reduction:
The first parameter is a timing of starting the pressure reduction.
The second parameter is a quantity of pressure reduction.
The third parameter is a rate of change in pressure reduction.
The following describes the change of the dot creation state by varying the pressure waveform with regard to the various parameters. FIG. 4 shows ejection of an ink droplet in response to a driving waveform applied to the print head. In this illustrated example, the waveform to drive the print head first lowers the pressure in a division d1, raises the pressure in a division d2, and lowers the pressure in a division d4 again after the elapse of a division d3. The divisions d2 through d4 correspond to the xe2x80x98waveform applying a high pressure to the ink and subsequently reducing the pressurexe2x80x99 discussed above.
The waveform of FIG. 4 represents a standard state without varying any of the first through the third parameters defined above. States xe2x80x98axe2x80x99 through xe2x80x98cxe2x80x99 show the behavoir of ink when the print head is actuated in response to the standard waveform. The states xe2x80x98axe2x80x99 through xe2x80x98cxe2x80x99 are enlarged sectional views of the nozzle Nz formed in the print head. When the pressure is reduced in the division d1, the interface of ink or meniscus is concaved inward due to the pressure variation as shown in the state xe2x80x98axe2x80x99.
When the pressure is raised in the subsequent division d2, the raised pressure causes an ink droplet Ip to be ejected as shown in the state xe2x80x98bxe2x80x99. The ink droplet Ip is ejected from the substantial center of the meniscus Me, which is kept in the concaved state. When the pressure is reduced again in the division d4, the vibration arising at the meniscus at the moment of ink ejection is damped and the meniscus is returned to the original state prior to the ejection. The ejected ink droplet Ip flies to hit against the printing medium to create a dot DL.
FIG. 5 shows a variation in pressure waveform with a change of the first parameter. The first parameter is the timing of pressure reduction. The change of the parameter is equivalent to the change of the time period prior to the pressure reduction after application of a high pressure, from a period d3a to a period d3c in the illustrated example. The pressure waveform varies in three steps expressed as waveform segments L3a, L3b, and L3c in the order of the timing. These waveform segments have an identical quantity of pressure reduction and an identical rate of change.
FIG. 6 shows the state of an ink droplet when the pressure is reduced at the earliest timing. This corresponds to the waveform segment L3a in FIG. 5. As the pressure decreases, the force is applied to the meniscus Me to draw the meniscus Me inward the nozzle Nz. The meniscus Me accordingly has a velocity component Vme drawn inward the nozzle. The velocity component Vme arising at the meniscus Me has the function of separating an ejected ink droplet Ip in an area Ir in the vicinity of the boundary. In the case where the pressure is reduced before the ink droplet Ip is completely separated from the meniscus Me, the surface tension of ink causes the ink droplet Ip to be affected by the velocity component Vme arising at the meniscus Me. The ink droplet Ip accordingly has a local speed difference. A front division Ipf of the ejected ink droplet flies at a relatively high speed, whereas a rear division Ipb of the ink droplet has a lower flight speed. The right side of FIG. 6 shows resulting dots thus created. When the meniscus Me is drawn inward the nozzle at a relatively early timing, the resulting dot is split into divisions as illustrated.
FIG. 7 shows the state of an ink droplet when the pressure is reduced at the intermediate timing. This corresponds to the waveform segment L3b in FIG. 5. Delaying the timing changes the state of separation of the ink droplet from the meniscus and the local speed difference of the ink droplet. In the case of the delayed timing, the meniscus Me is drawn inward the nozzle when the ink droplet Ip is further away the nozzle. There is accordingly a relatively small area having a locally reduced speed. Namely the rear division Ipb of the ink droplet has a small volume. As shown on the right side of FIG. 7, a small dot is created adjacent to a relatively large dot in this case.
FIG. 8 shows the state of an ink droplet when the pressure is reduced at the slowest timing. This corresponds to the waveform segment L3c in FIG. 5. Further delaying the timing causes the meniscus Me to be drawn inward the nozzle when the ink droplet Ip has almost been separated from the meniscus Me. The drawing action of the meniscus Me thus hardly affects the behavoir of the ink droplet Ip. As shown on the right side of FIG. 8, a single dot is accordingly created. Changing the timing of drawing the meniscus Me inward as discussed above enables creation of the split dot and adjustment of the size and the flight speed of the rear division of the split dot.
FIG. 9 is a graph showing the results of experiments varying the first parameter. Variations in flight speeds and areas of the respective divisions of the dot ejected in a splitting manner are plotted against the first parameter, that is, the time period d3 prior to the start of speed reduction, on the abscissa. The symbols Vf, Vb, Ipf, and Ipb have the same meanings as those of FIGS. 6 through 8. As shown in the graph, as the parameter d3 increases, the front division of the split dot has the increasing volume Ipf while keeping the flight speed Vf substantially constant. The rear division of the split dot, on the other hand, has the increasing flight speed Vb but the decreasing volume Ipb. When the parameter d3 exceeds a certain threshold value, the ink droplet is not split but forms a single dot. Areas F6, F7, and F8 shown in the graph respectively correspond to the states of FIGS. 6 through 8 discussed above.
The following describes the effects of the second parameter. FIG. 10 shows a variation in pressure waveform with a change of the second parameter. The second parameter is the quantity of pressure reduction. In this example, the quantity of pressure reduction after application of a high pressure is varied in three different stages. The quantity of pressure reduction increases in the order of waveforms L4a, L4b, and L4c. The waveforms L4b and L4c reduce the pressure to the lower levels than the reference pressure before the ink ejection. In such cases, after the ejection of an ink droplet is concluded, the pressure of the ink is returned to the reference level at a specific rate that does not cause ink to be ejected from the nozzle.
FIG. 11 shows the state of an ink droplet under the condition of a smallest quantity of pressure reduction. This corresponds to the waveform L4a in FIG. 10. FIG. 12 shows the state of an ink droplet under the condition of an intermediate quantity of pressure reduction. This corresponds to the waveform L4b in FIG. 10. FIG. 13 shows the state of an ink droplet under the condition of a largest quantity of pressure reduction. This corresponds to the waveform L4c in FIG. 10.
The effects of the inwardly drawn meniscus Me are described previously with FIGS. 5 through 8. The variation in quantity of pressure reduction changes the drawing depth of the meniscus Me. This varies the velocity of the part of the ink droplet affected by the inwardly drawn meniscus Me. The greater drawing depth of the meniscus Me results in the lower velocity of the rear division Ipb of the split dot as clearly understood from FIGS. 11 through 13. As shown in FIGS. 11 through 13, the increase in drawing depth of the meniscus Me enhances the velocity component Vme of the rear division of the ink droplet (that is, the left division of the split dot in the drawing) drawn inward the nozzle, but lowers the velocity of the rear division of the ink droplet after the separation of the ink droplet from the meniscus Me. As shown on the right side of FIGS. 11 through 13, this extends the interval between the divisions of the split dot. Although FIGS. 11 through 13 show the split dots clearly divided into two parts, the two divisions of the split dot may be overlapped.
FIG. 14 is a graph showing the results of experiments varying the first parameter. Variations in flight speeds and areas of the respective divisions of the dot ejected in a splitting manner are plotted against the second parameter, that is, the quantity of pressure reduction, on the abscissa. As shown in the graph, as the quantity of pressure reduction increases, the front division of the split dot has the increasing volume Ipf while keeping the flight speed Vf substantially constant. The rear division of the split dot, on the other hand, has both the decreasing flight speed Vb and the decreasing volume Ipb. Areas F11, F12, and F13 shown in the graph respectively correspond to the states of FIGS. 11 through 13 discussed above.
The following describes the effects of the third parameter. FIG. 15 shows a variation in pressure waveform with a change of the third parameter. The third parameter is a rate of change in pressure reduction. Here the rate of change denotes the quantity of reduction per unit time. In this example, the rate of change in pressure reduction after application of a high pressure is varied in three different stages. The rate of change in pressure reduction decreases in the order of waveforms L4d, L4e, and L4f. 
FIG. 16 shows the state of an ink droplet under the condition of a largest rate of change in pressure reduction. This corresponds to the waveform L4d in FIG. 15. FIG. 17 shows the state of an ink droplet under the condition of an intermediate rate of change in pressure reduction. This corresponds to the waveform L4e in FIG. 15. FIG. 18 shows the state of an ink droplet under the condition of a largest rate of change in pressure reduction. This corresponds to the waveform L4f in FIG. 17.
The rate of change in pressure reduction affects the drawing velocity of the meniscus Me. The smaller rate of change in pressure reduction results in the lower drawing velocity Vme of the meniscus Me as clearly understood from FIGS. 16 through 18. The ejected ink droplet Ip accordingly has a smaller local speed difference. As shown in FIGS. 16 through 18, the smaller local speed difference causes the front division Ipf and the rear division Ipb of the ink droplet to have the closer hitting positions on the printing medium. Although FIG. 18 schematically shows two dots formed close to each other in a partly overlapping manner, one elliptical dot having a major axis from left to right is actually formed by the effect of blotting. In the specification hereof, a deformed dot due to a local speed difference of the ink droplet is regarded as one state of xe2x80x98divisionxe2x80x99.
FIG. 19 is a graph showing the results of experiments varying the third parameter. Variations in flight speeds and areas of the respective divisions of the dot ejected in a splitting manner are plotted against the third parameter, that is, the rate of change in pressure reduction, on the abscissa. As shown in the graph, as the rate of change in pressure reduction increases, that is, as the pressure is abruptly reduced, the front division of the split dot keeps both the flight speed Vf and the volume Ipf substantially constant. The rear division of the split dot, on the other hand, has the decreasing flight speed Vb while keeping the volume Ipb substantially constant. Areas F16, F17, and F18 shown in the graph respectively correspond to the states of FIGS. 16 through 18 discussed above.
As described above, the dot creation state is adjusted to create, for example, a non-split dot, a split-dot, or a deformed dot by varying any of the parameters relating to the pressure reduction after the ejection of the ink droplet Ip. Varying the parameters also regulates the interval between the divisions of the split dot and the volumes of the front division and the rear division of the split dot. Each of the parameters discussed above may be varied while the pressure relating to the ejection of the ink droplet (that is, the division d2 in FIG. 4) is kept constant. The dot creation state is thus changed appropriately under the condition of a fixed quantity of ink by varying each of the parameters discussed above.
With the reference waveform shown in FIG. 4, the following describes the case where the preset pressure waveform includes a high pressure section to apply a high pressure to the ink and a pre-reducing pressure section to reduce the pressure prior to the high pressure section, and the parameter is a quantity of pressure reduction in the pre-reducing pressure section.
The divisions d1 to d2 in FIG. 4 correspond to the xe2x80x98waveform of reducing the pressure prior to application of a high pressure to the inkxe2x80x99. FIG. 20 shows a variation in pressure waveform with a change of the quantity of pressure reduction in the division d1. This example shows two waveforms respectively corresponding to a small quantity of pressure reduction (waveform L1a) and a large quantity of pressure reduction (waveform L1b). With regard to the pressure at the time of ink ejection (corresponding to the division d2 in FIG. 4), the waveforms L1a and L1b may have an identical peak pressure or an identical pressure difference. In the case of the identical peak pressure, the waveforms L1a and L1b commonly follow a waveform segment L2a shown in FIG. 20. In the case of the identical pressure difference, on the other hand, the waveform L1a follows the waveform segment L2a whereas the waveform L1b follows another waveform segment L2b. 
FIG. 21 shows the state of the meniscus Me according to the variation in quantity of pressure reduction. The left-side drawing shows the state corresponding to the small quantity of pressure reduction (the waveform L1a in FIG. 20). The right-side drawing shows the state corresponding to the large quantity of pressure reduction (the waveform L1b 9 in FIG. 20). In the case of the small quantity of pressure reduction, the meniscus Me is concaved inward the nozzle Nz in response to the pressure reduction as shown by the left-side drawing.
As the quantity of pressure reduction increases, the meniscus Me is concaved to a greater depth and has a rise S on the substantial center thereof as shown by the right-side drawing. The cause of this phenomenon has not been elucidated clearly. But it is assumed that abruptly drawing the meniscus inward the nozzle destroys the balance of the surface tension of the meniscus and a vibration arises to invert the substantial center of the meniscus having the smallest in surface tension. The occurrence of the rise S affects the velocity of the ejected ink droplet.
FIG. 22 shows the state of an ink droplet in the case of the small quantity of pressure reduction. This corresponds to the waveform L1a in FIG. 20. FIG. 23 shows the state of an ink droplet in the case of the large quantity of pressure reduction. This corresponds to the waveform L1b in FIG. 20. As described previously, varying the quantity of pressure reduction changes the velocity of the meniscus Me immediately before the ink ejection. The case of the large quantity of pressure reduction (the right-side drawing of FIG. 21) has a higher velocity component on the substantial center of the meniscus in the direction of ejection (that is, the direction Dir in FIG. 21), so that the ink droplet is ejected at a higher velocity. As clearly understood from FIGS. 22 and 23, varying the quantity of pressure reduction thus regulates the velocity of the front division Ipf of the ink droplet and the interval between the respective divisions of the split dot.
FIG. 24 is a graph showing the results of experiments when the quantity of speed reduction is changed while the peak pressure is kept constant. Variations in flight speeds of the respective divisions of the dot ejected in a splitting manner are plotted against the quantity of pressure reduction on the abscissa. As shown in the graph, under the condition of small quantities of pressure reduction, the ink droplet is not split but forms a single dot. When the quantity of pressure reduction exceeds a predetermined threshold value, the ink droplet is split into a front division and a rear division. With an increase in quantity of pressure reduction, there is a greater speed difference between the front division and the rear division of the split dot. Areas F22 and F23 shown in the graph respectively correspond to the states of FIGS. 22 and 23 discussed above.
As described above, the dot creation state is varied by generating the waveform that includes a pressure reducing section at least either before or after the ink ejection and regulating the pressure reduction. The waveform may include the pressure reducing sections both before and after the ink ejection.
The pressure reduction may be varied in a continuous manner, instead of in the stepwise manner as in the above description. Appropriate values are set to the respective parameters according to the nozzle diameter and the viscosity of ink, in order to attain a desirable dot creation state for printing. The above description regards the primary effects of the respective parameters on the dot creation state. In the actual conditions, the respective parameters closely affect one another.
In accordance with one preferable application of the print head of the present invention, the pressure variation unit changes a cross section of the ink conduit, so as to vary the pressure applied to the ink.
It is especially preferable that the pressure variation unit includes an electrostrictive element that is disposed adjacent to the ink conduit to generate a predetermined strain in response to an applied voltage, and that the driving unit regulates the voltage applied to the electrostrictive element to vary the pressure. A piezoelectric element may be applied for the electrostrictive element.
Such application for the pressure variation unit ensures a variation in ink pressure with a high response, thus enabling the print head to be driven at a high frequency. The print head of this application ensures printing of the high image quality, while keeping the high printing speed.
The print head having any of the above configurations may create a plurality of different dots having different quantities of ink. For example, in a print head that changes the quantity of ink ejected per pixel between a high level and a low level, dots may be created in the plurality of dot creation states discussed above with regard to only one of the two different levels of ink quantity or with regard to both the two different levels of ink quantity.
The present invention is also directed to a printing apparatus that ejects ink and creates a dot in each pixel on a printing medium, so as to print a multi-tone image.
The printing apparatus includes: an input unit that inputs halftone-processed print data; and a dot creation unit that selectively uses one of a plurality of preset different dots according to the input print data and creates the selected dot in each pixel.
The plurality of preset different dots include at least two different dots corresponding to a plurality of dot creation states having different areas with a substantially equivalent quantity of ink.
The printing apparatus of this arrangement enables creation of dots in the plurality of dot creation states having different areas. As described previously, the resulting expressed density depends upon the dot area. The printing apparatus of the present invention accordingly enables a plurality of different densities to be expressed with regard to each pixel, thus ensuring the smooth tone expression and improving the image quality of printing. These effects are especially prominent in the low tone area.
The technique of varying the quantity of ink ejected in each pixel is the only proposed technique as the prior art to express a plurality of different densities with a fixed quantity of ink. All the other proposed techniques mentioned above, for example, the technique of varying the frequency of ink ejection in each pixel and the technique of varying the quantity of ink per ejection, express the multiple densities by varying the total quantity of ink ejected in each pixel.
As discussed above, however, the resulting expressed density may be varied by changing the dot creation state under the condition of a substantially identical quantity of ink. The printing apparatus of the present invention is based on this principle to attain the smooth tone expression. This arrangement is free from the disadvantages of a large-sized print head provided with a large number of different inks having different densities, although the present invention does not exclude the structure using inks of different densities. The application of creating dots in a plurality of dot creation states as described previously with inks of different densities attains smoother tone expression.
The printing apparatus of the present invention can vary the dot area without changing the quantity of ink ejected in each pixel. This arrangement thus varies the expressible tone value in each pixel, regardless of the restriction of the absorbable quantity of ink per unit area of the printing medium (hereinafter referred to as the duty restriction). This arrangement ensures the smooth tone expression even in printing media of low duty restriction.
In accordance with a first configuration of the printing apparatus of the present invention, the print head is applied for the dot creation unit described above.
In accordance with a second configuration, the dot creation unit has: an ink ejection unit that is capable of varying a quantity of ink per ejection; and a driving unit that controls the ink ejection unit to change a quantity, a frequency, and a position of ink ejection and thereby ensure creation of dots in the plurality of dot creation states.
For example, in the case of creating a split dot having two divisions as shown in FIG. 2, this application halves the quantity of ink per ejection and ejects ink twice at different positions. The printing apparatus generally carries out printing while moving the print head back and forth relative to the printing medium (hereinafter referred to as the main scan). In this structure, ejection of ink at two different times having a predetermined time interval enables two dots to be created at different positions. The two dots may be created by one pass of the main scan or by two different passes of the main scan. The above description regards the case of creating a split dot having two divisions, but the same principle is applied to create a split dot having a greater number of divisions.
The second structure has an advantage of stably creating a split dot. A variety of mechanisms have been proposed to vary the quantity of ink per ejection. For example, in a print head based on the mechanism of supplying electricity to heaters disposed in the nozzles and ejecting ink by means of the pressure of bubbles produced in the ink, the quantity of ink per ejection is varied by regulating the number of heaters and the quantity of power supply. In another print head based on the mechanism of ejecting ink by means of the strain arising in the process of application of a voltage to piezoelectric elements, the quantity of ink per ejection is varied by changing the waveform of the voltage applied. The technique of the present invention is not restricted to these print heads but is applicable to a variety of other print heads that are capable of varying the quantity of ink.
It is preferable that the printing apparatus described above enables expression of three or more density values. Each tone value after the halftone processing to the three or more density values corresponds to the evaluation value of the density expressed by each dot in each pixel. The halftone processing is not necessarily performed in the printing apparatus, but the printing apparatus may receives the halftone-processed data and carry out printing. The printing apparatus may alternatively process the multi-tone image data by halftone processing and then carry out printing. A variety of methods, such as the error diffusion method and the dither method, are applicable for the halftone processing.
The present invention is further directed to a printing apparatus that creates dots on a printing medium, so as to print a multi-tone image.
The printing apparatus includes: an input unit that inputs print data halftone-processed to a preset number of tone values; a dot creation state changing unit that changes over a dot creation state among different dot creation states having different densities expressed with a substantially identical quantity of ink; a printing medium input unit that inputs a type of a printing medium; a storage unit that stores a mapping of each tone value of the print data to each of the different dot creation states with regard to each printing medium; and a control unit that controls the dot creation state changing unit based on the mappings stored in the storage unit and enables dots to be created in a selected dot creation state according to the input type of the printing medium.
The printing apparatus of this arrangement enables creation of dots in a dot creation state suitable for the type of the printing medium. Different printing media generally have different penetration characteristics in the course of ink ejection and accordingly have different resulting densities expressed with dots created by ejecting a fixed quantity of ink. The printing apparatus of the above configuration changes the dot creation state corresponding to the printing medium and thereby compensates for a density difference due to the different ink penetration characteristics of the printing media. This arrangement ensures the appropriate tone expression suitable for each printing medium.
The effects of the improved image quality are especially prominent in printing media of low duty restriction. The printing media of low duty restriction generally have a high ink penetration speed. The ejected ink thus quickly penetrates in the direction of the depth of the printing medium, and the dye of the ink is not sufficiently held in the vicinity of the surface to ensure the sufficient density expression. The low duty restriction also makes it impossible to increase the quantity of ink to allow the sufficient density expression. The printing apparatus of the present invention changes the dot creation state to enhance the expressible density without increasing the quantity of ink. This arrangement enables the sufficient tone expression even in printing media of the low duty restriction and accordingly improves the image quality.
As described previously with FIG. 25, the resulting expressed density varies with a variation in number of divisions of the split dot under the condition of a fixed quantity of ink. FIG. 18 shows the comparison on the same printing medium. The dot creation state on the printing medium having the high ink permeability is compared with the dot creation state on the printing medium having the restricted ink permeability. When a fixed quantity of ink is ejected, the dot created on the former printing medium has a greater area than that of the dot created on the latter printing medium. The ink, however, penetrates in the direction of the depth of the former printing medium, so that the former dot has a lower expressed density than that of the latter dot. Namely the relationship between the dot area and the resulting expressed density depends upon the printing medium.
The graph of FIG. 25 shows that the split dot has a higher expressed density than the single dot on the same printing medium. Under the condition of a fixed quantity of ink ejection, the technique creates the split dot on the printing medium of the high ink permeability and the single dot on the printing medium of the restricted ink permeability. This arrangement reduces the difference in density expression between these printing media. The printing apparatus of the present invention attains the appropriate tone expression with regard to each printing medium, based on this principle.
The printing apparatus of the present invention is characterized by the relationship between the print data and the dot to be created that is different from that of the prior art technique. The prior art printing apparatus typically has a fixed relationship between the print data and the dot to be created, regardless of the type of the printing medium. For example, in the case of the printing apparatus that is capable of binary density expression, that is, the dot-on and the dot-off states, in each pixel, the same dot is created according to the print data representing the dot-on state, regardless of the type of the printing medium. The printing apparatus that is capable of at least trinary density expression in each pixel follows the same principle. The prior art technique changes the dot recording density according to the type of the printing medium, so as to compensate for the density difference due to the difference in ink permeability.
The printing apparatus of the present invention, on the other hand, varies the relationship between the print data and the dot to be created according to the printing medium. The technique of the present invention creates dots in different dot creation states corresponding to different printing media with regard to the print data representing the dot-on state. In the case where the density difference due to the difference in ink permeability is sufficiently compensated by the change of the dot creation state, printing may be carried out using the common print data, irrespective of the type of the printing medium. Combination of the technique of varying the dot recording density with the technique of changing the dot creation state according to the printing medium, however, enables the density difference to be compensated more appropriately.
It is desirable that the mapping stored in the storage unit maps a dot creation state attaining expression of a higher density to a printing medium having a lower quantity of ink absorbable per unit area.
The ink readily permeates the printing medium having the low quantity of ink absorbable per unit area, that is, having the low duty restriction, in the direction of its depth. Such a printing medium accordingly tends to have the low expressed density. Setting the dot creation state to enhance the expressed density, for example, with a split dot, in this printing medium desirably reduces the difference in expressed density between the respective printing media, thus ensuring the appropriate tone expression.
The mapping of the dot creation state to the type of the printing medium is not restricted to the above description. A variety of settings are applicable to attain the appropriate tone expression by taking into account the ink permeability of each printing medium. It is not necessary that all the printing media have different dot creation states.
In the printing apparatus of the present invention, a variety of configurations may be applied for the dot creation state changing unit.
In accordance with a first configuration, the dot creation state changing unit changes a number of dot divisions from an ejected ink droplet, so as to enable dots to be created in the different dot creation states having different densities. This corresponds to the structure using the print head described previously. The number of dot divisions here includes the value xe2x80x981xe2x80x99 representing a non-split dot. A variety of methods may be applied to create a split dot according to the mechanism of the print head for ejecting ink. For example, a sub nozzle used only for creation of a split dot may be disposed adjacent to the nozzle for ejecting ink. Another method applies a vibration to the nozzle at the time of ink ejection.
It is not necessary that the divisions of the split dot are created at the same time. For example, the split dot may be created by forming two small dots in one pixel at two different times with half the quantity of ink.
In accordance with a second configuration, the dot creation state changing unit gives a local speed difference to an ink droplet ejected, so as to change the dot creation state.
This configuration may be applied to create a split dot. Ejection of an ink droplet with a local speed difference changes the shape of the ink droplet according to the degree of the speed difference and enables creation of dots in various states. In the case of a large speed difference, a split dot is created. The local speed difference is caused by varying the pressure applied to the ink in the course of ejection. For example, when the pressure is raised at the initial stage of the ink ejection and is lowered at the terminal stage, the part of the ejected ink droplet closer to the nozzle has the lower flight speed.
In accordance with a third configuration, the dot creation state changing unit varies a distance between the print head and the printing medium, so as to change the dot creation state.
The third configuration may also be applied to create a split dot. The ink droplet is deformed by the air resistance during the flight. In the case where the print head is close to the printing medium, the air resistance works only for a short time period, so as to cause a relatively small degree of deformation. With an increase in distance between the print head and the printing medium, the working time of the air resistance is lengthened to increase the degree of deformation. In some cases, the ink droplet is split into two or more divisions. The dot creation state is accordingly changed by varying the distance between the print head and the printing medium.
In accordance with a fourth configuration, when the printing apparatus includes a main scan unit that moves back and forth the print head relative to the printing medium to carry out main scan in the course of printing, the dot creation state changing unit varies a moving speed of the main scan, so as to change the dot creation state.
The fourth configuration may also be applied to create a split dot. As described above in the third configuration, the ink droplet is deformed by the air resistance during the flight. The air resistance working on the ink droplet is affected by the composite velocity of the ejecting speed of the ink droplet and the moving speed of the print head. As is generally known, the air resistance increases proportionally to the second power of the velocity. Changing the air resistance applied to the ink droplet varies the degree of deformation of the ink droplet by the air resistance. The dot creation state is accordingly changed by varying the moving speed of the print head.
The principle of the present invention is attained by a variety of applications, for example, the method of driving the print head, and the printing method, other than the applications described above. The technique of the present invention may be constructed as a program for driving the print head or the printing apparatus, a variety of signals equivalent to this program, and a recording medium in which such a program is recorded. Available examples of the recording medium include flexible disks, CD-ROMs, magneto-optic discs, IC cards, ROM cartridges, punched cards, prints with barcodes or other codes printed thereon, internal storage devices (memories like a RAM and a ROM) and external storage devices of the computer, and a variety of other computer readable media.